MEMS devices have a wide variety of applications and are becoming more prevalent in commercial products. MEMS devices are ideal for wireless devices because of their low power and loss along with high isolation and linearity characteristics operating in radio frequency (RF) ranges. In particular, MEMS devices are well suited for applications including cellular telephones, wireless networks, communication systems, and radar systems. In wireless devices, MEMS devices can be used as antenna switches, mode switches, transmit/receive switches, tunable filters, matching networks and the like.
MEMS devices (also known as micromachines) are generally classified into two groups according to their manufacturing techniques. One is called a bulk micromachine which is obtained by manufacturing a three-dimensional structure in such a way that a silicon wafer or a SOI (Silicon On Insulator) substrate itself is processed by etching or polishing. The other is called a surface micromachine which is obtained by manufacturing a three-dimensional structure in such a way that a thin film is stacked over a substrate such as a silicon wafer and the thin film is processed by photolithography and etching.
Surface micromachined MEMS devices can have reproducibility and reliability concerns that vary with the processes used to fabricate them. One problem is associated with the use of chemical etchants, which can lead to rough features on the surfaces of processed metal structures. These rough features on metal structures are known as hillocks. Hillocks can grow due to grain boundary slippage and compressive stress. Such growth is typically driven by thermal history at elevated temperatures during processing where the metal structures are in compression since they expand more rapidly than the substrate, insulator or other structure to which the metal structures are attached or in which the metal structures are buried. The elevated temperatures can also cause the metal grains to coalesce into larger grains. Since the metals are in compression, if grain slippage occurs, it relieves this compression by displacing outward to make space. This displacement is on the order of the grain size and can form a hillock.
The presence of hillocks on MEMS devices can prevent proper device operation. For example, hillocks can increase leakage and breakdown reliability where hillocks are field intensifiers in high voltage regions of the MEMS device. Further, hillocks create natural stress concentration points that are more likely to fragment over time, which can create destructive free particulates in the MEMS device. If a MEMS device closes on a hillock, this may cause fragmentation. Also, if the hillock is present in a gap between two element of the MEMS device, it may limit the motion and thus the function of the MEMS device. This is particularly important in capacitive RF MEMS devices where very small gaps are required for optimal function. As a result, it is highly desirable to reduce or eliminate hillocks. Accordingly, in light of these difficulties, there exists a need to improve MEMS metal structures and related formation techniques for reducing or eliminating hillock formation.